Method for measuring complex degree of coherence of random optical field by using mutual intensity-intensity correlation

ABSTRACT

The invention discloses a method for measuring a complex degree of coherence of a random optical field by using a mutual intensity-intensity correlation, including the steps of: building a test optical path; rotating a quarter-wave plate to enable the fast axis of the quarter-wave plate to be consistent with a polarization direction of reference light, to obtain light intensity distribution information of a first combined light; rotating the quarter-wave plate to enable the slow axis of the quarter-wave plate to be consistent with the polarization direction of the reference light, to obtain light intensity distribution information of a second combined light; blocking the reference light to obtain light intensity distribution information of to-be-tested light; blocking the to-be-tested light to obtain light intensity distribution information of the reference light; and calculating the amplitude and phase of a complex degree of coherence of the to-be-tested light.

FIELD OF THE INVENTION

The present invention relates to the field of optical field measurementtechnology, and particularly to a method for measuring a complex degreeof coherence of a random optical field by using a mutualintensity-intensity correlation.

DESCRIPTION OF THE RELATED ART

Because of random fluctuations in the natural world, any optical fieldis inevitably accompanied with some random fluctuations in physicalparameters such as light intensity, phase, and polarization. Such arandom optical field is usually also referred to as a partially coherentoptical field. Compared with a conventional fully coherent opticalfield, the partially coherent optical field has a higher degree offreedom, for example, a space-time coherence and a coherence structuredistribution. It is found through the research in the recent decade thatnovel physical effects such as self-splitting, self-shaping, andself-healing may be generated by modulating the coherence structure ofthe partially coherent optical field, so that a partially coherent beamwith a novel coherence structure distribution has important applicationin fields such as micro/nano manipulation, complex system opticalcommunication, and novel optical imaging. For a random optical field, aspatial coherence of an optical field is a fundamental property of anelectromagnetic wave, and is vital to the comprehension of theinterference and transmission of classical and quantum optical waves andthe interaction between matters. The spatial coherence of the opticalfield describes a spatial degree of coherence between two or more pointsof the optical field in a space. If the degree of coherence is higher,the spatial coherence of the optical field is better, and if the degreeof coherence is lower, the spatial coherence is poorer. In the 50s and60s in the last century, Wolf et al. proposed a method forquantitatively describing a spatial coherence of a random optical field,and provided the definition of a spatial degree of coherence. In thedefinition, the spatial degree of coherence is a complex function, andthe absolute value of the complex function is between 0 and 1. When thespatial degree of coherence is equal to 0, it indicates that in thiscase there is no correlation between two points in a space. When thespatial degree of coherence is equal to 1, it indicates that in thiscase two points are fully correlated. The modulation of the coherencestructure of the partially coherent optical field in the past researchis mostly the modulation of the amplitude of a degree of coherence.

Currently, researchers have proposed various methods for measuring acomplex degree of coherence of a random optical field. These methods mayinclude a method of optical field interference and a method ofintensity-intensity correlation. In the method of an optical fieldinterference, a to-be-tested light field is divided into two parts,coherent superposition is performed to form an interference pattern, andcomplex degree of coherence information of the to-be-tested light fieldis eventually recovered from the interference pattern. The mostclassical method is the method of Young's double slit interference. Inthe method, to-be-tested light passes through two circular pinholeslocated at a particular interval, and interference pattern distributionin the far field or in the focal plane is then measured to recover acomplex degree of coherence of the incident random optical field. Theamplitude information of the complex degree of coherence can be obtainedby calculating the visibility of the interference pattern, and phaseinformation of the complex degree of coherence can be inversely derivedby using a relative displacement between two measurements of theinterference pattern. However, the solution has two critical defectsduring actual application. First, for the measurement of two-dimensionaldegree of coherence distribution of an optical field, a large quantityof far field interference patterns of two holes with different intervalsneed to be measured, consuming a long time. Secondly, to ensure thespatial resolution of the measured degree of coherence, the opening sizeof a pinhole is generally very small, thus the optical efficiency of asystem is restricted, and an error in a measurement process isincreased. For the method of an optical field intensity-intensitycorrelation, only the amplitude of the complex degree of coherence canbe directly measured, the phase information is lost during measurement,and it is very time consuming to recover the phase information in alater stage.

SUMMARY OF THE INVENTION

The technical problem to be resolved by the present invention is toprovide a method for measuring a complex degree of coherence of a randomoptical field by using a mutual intensity-intensity correlation, so thatthe fast and high-resolution measurement of the amplitude and phase of acomplex degree of coherence for a random optical field can beimplemented.

To solve the foregoing technical problem, the present invention providesa method for measuring a complex degree of coherence of a random opticalfield by using a mutual intensity-intensity correlation, including thefollowing steps:

building a test optical path including a quarter-wave plate, a beamsplitter, a condensing element and a light detector and using a laser asreference light, where the reference light is modulated by thequarter-wave plate and then enters the beam splitter, and to-be-testedlight enters the beam splitter at the same time, the beam splittercombines the modulated reference light and the to-be-tested light toobtain combined light, and the combined light passes through thecondensing element to be imaged on the light detector;

rotating the quarter-wave plate to enable the fast axis of thequarter-wave plate to be consistent with a polarization direction of thereference light, to obtain first combined light, and photographing andrecording light intensity distribution information I_(s) ⁽¹⁾(r) of thefirst combined light by using the light detector;

rotating the quarter-wave plate to enable the slow axis of thequarter-wave plate to be consistent with the polarization direction ofthe reference light, to obtain second combined light, and photographingand recording light intensity distribution information I_(s) ⁽²⁾(r) ofthe second combined light by using the light detector;

blocking the reference light, and photographing and recording lightintensity distribution information I(r) of the to-be-tested light byusing the light detector;

blocking the to-be-tested light, and photographing and recording lightintensity distribution information S_(r)(r) of the reference light byusing the light detector; and

calculating the amplitude and phase of a complex degree of coherence ofthe to-be-tested light.

Preferably, the “calculating the amplitude and phase of a complex degreeof coherence of the to-be-tested light” specifically includes thefollowing steps:

S61. calculating a mutual correlation G_(S) ^((1,2))(r₁,r₂) between thelight intensity distribution information I_(S) ⁽¹⁾(r) of the firstcombined light and the light intensity distribution information I_(S)⁽²⁾(r) of the second combined light;

S62. calculating a mutual correlation G_(B) ^((1,2))(r₁,r₂) obtained byadding the reference light and the to-be-tested light;

S63. calculating a difference value ΔG^((1,2))(r₁,r₂,Δϕ) between the twomutual correlations G_(S) ^((1,2))(r₁,r₂) and G_(B) ^((1,2))(r₁,r₂); and

S64. analyzing the difference value ΔG^((1,2))(r₁,r₂,Δϕ) between the twomutual correlations, to obtain the amplitude and phase of the complexdegree of coherence of the to-be-tested light.

Preferably, S61 specifically includes:

according to a second-order coherence matrix in the space-frequencydomain, representing, by using a cross-spectral density function, asecond-order statistical feature of the to-be-tested light as:

W(r ₁ ,r ₂)=

E*(r ₁)E(r ₂)

  (1),

where E(r) represents a random electrical field at a point r in a space,a superscript asterisk represents a complex conjugate, an angle bracketrepresents ensemble averaging, and in this case, a complex spatialdegree of coherence between two points r₁ and r₂ in the space may bedefined as:

$\begin{matrix}{{{\mu\left( {r_{1},r_{2}} \right)} = \frac{W\left( {r_{1},r_{2}} \right)}{\sqrt{{S\left( r_{1} \right)}{S\left( r_{2} \right)}}}},} & (2)\end{matrix}$

where S(r)=W(r, r)=

I(r)

represents average light intensity of a random optical field at thepoint r in the space;

in the step of “rotating the quarter-wave plate to enable the fast axisof the quarter-wave plate to be consistent with a polarization directionof the reference light”, an electrical field of the reference lightobtained through modulation by using the quarter-wave plate is denotedas E_(r) ⁽¹⁾(r), in the step of “rotating the quarter-wave plate toenable the slow axis of the quarter-wave plate to be consistent with thepolarization direction of the reference light”, an electrical field ofthe reference light obtained through modulation by using thequarter-wave plate is denoted as E_(r) ⁽²⁾(r), and an electrical fieldof the to-be-tested light is denoted as E(r);

there is a phase difference of

$\frac{\pi}{2}$

between the electrical fields E_(r) ⁽¹⁾(r) and E_(r) ⁽²⁾(r), that is:

$\begin{matrix}{{{\Delta\phi} = {{{{Arg}\left\lbrack {E_{r}^{(1)}(r)} \right\rbrack} - {{Arg}\left\lbrack {E_{r}^{(2)}(r)} \right\rbrack}} = \frac{\pi}{2}}},} & (3)\end{matrix}$

where Arg represents calculating the phase of a complex function;

an electrical field E_(S) ⁽¹⁾(r) of the first combined light and anelectrical field E_(S) ⁽²⁾(r) of the second combined light arerepresented as follows:

E _(S) ⁽¹⁾(r)=E(r)+E _(r) ⁽¹⁾(r)   (4),

E _(S) ⁽²⁾(r)=E(r)+E _(r) ⁽²⁾(r)   (5),

the mutual correlation between the light intensity distributioninformation I_(S) ⁽¹⁾(r) of the first combined light and the lightintensity distribution information I_(S) ⁽²⁾(r) of the second combinedlight is:

G _(S) ^((1,2))(r ₁ ,r ₂)=

I _(S) ⁽¹⁾(r ₁)I _(S) ⁽²⁾(r ₂)

  (6), and

according to theorem of Gaussian statistics, Formula (1) to Formula (5)are substituted into Formula (6), to obtain:

G _(S) ^((1,2))(r ₁ ,r ₂)=G _(S) ^((1,2))(r ₁ ,r ₂,Δϕ)=S _(S) ⁽¹⁾(r ₁)S_(S) ⁽²⁾(r ₂)+|W(r ₁ ,r ₂)|²+2√{square root over (S _(r) ⁽¹⁾(r ₁)S _(r)⁽²⁾(r ₂))}Re[e ^(iΔϕ) W(r ₁ ,r ₂)]  (7).

Preferably, S62 specifically includes:

$\begin{matrix}\begin{matrix}{{G_{B}^{({1,2})}\left( {r_{1},r_{2}} \right)} = \left\langle {\left\lbrack {{S_{r}^{(1)}\left( r_{1} \right)} + {I\left( r_{1} \right)}} \right\rbrack\left\lbrack {{S_{r}^{(2)}\left( r_{2} \right)} + {I\left( r_{2} \right)}} \right\rbrack} \right\rangle} \\{= \left. {{{S_{S}^{(1)}\left( r_{1} \right)}{S_{S}^{(2)}\left( r_{2} \right)}} +} \middle| {W\left( {r_{1},r_{2}} \right)} \middle| {}_{2}. \right.}\end{matrix} & (8)\end{matrix}$

Preferably, S63 specifically includes:

ΔG _(S) ^((1,2))(r ₁ ,r ₂,Δϕ)=G _(S) ^((1,2))(r ₁ ,r ₂,Δϕ)−G _(B)^((1,2))(r ₁ ,r ₂)=√{square root over (S _(r) ⁽¹⁾(r ₁)S _(r) ⁽²⁾(r₂))}Re[e ^(iΔϕ) W(r ₁ ,r ₂)]  (9).

Preferably, S64 specifically includes:

for ΔG^((1,2))(r₁,r₂,Δϕ), taking a distribution Δϕ to be 0 and

$\frac{\pi}{2},$

and at the same time obtaining the real part and the imaginary part ofthe complex degree of coherence according to Formula (2):

$\begin{matrix}{{{{Re}\left\lbrack {\mu\left( {r_{1},r_{2}} \right)} \right\rbrack} = \frac{\Delta{G^{\langle{1,2})}\left( {r_{1},r_{2},{{\Delta\phi} = 0}} \right)}}{2\sqrt{{S_{r}^{(1)}\left( r_{1} \right)}{S_{r}^{(2)}\left( r_{2} \right)}{S\left( r_{1} \right)}{S\left( r_{2} \right)}}}},} & (10) \\{and} & \; \\{{{{Im}\left\lbrack {\mu\left( {r_{1},r_{2}} \right)} \right\rbrack} = \frac{\Delta{G^{({1,2})}\left( {r_{1},r_{2},{{\Delta\phi} = \frac{\pi}{2}}} \right)}}{2\sqrt{{S_{r}^{(1)}\left( r_{1} \right)}{S_{r}^{(2)}\left( r_{2} \right)}{S\left( r_{1} \right)}{S\left( r_{2} \right)}}}},} & (11)\end{matrix}$

in the foregoing formulas, Im represents calculating the imaginary partof the complex function, and obtaining the amplitude and phase of thecomplex degree of coherence of the to-be-tested light by using Formula(10) and Formula (11).

Preferably, the light detector is a charge-coupled device (CCD) or acomplementary metal-oxide-semiconductor (CMOS).

Preferably, the reference light is emitted by a helium-neon laser.

As compared with the prior art, the invention has the followingbeneficial effects:

1. The present invention achieves the fast and high-resolutionmeasurement of the amplitude and phase of a complex degree of coherenceof a random optical field by using a mutual intensity-intensitycorrelation.

2. There is no pinhole diffraction element in the method of the presentinvention. Therefore, a measurement system in the method has highoptical efficiency.

3. By means of the method for measuring a complex degree of coherence ofa random optical field by using a mutual intensity-intensity correlationin the present invention, the amplitude distribution of a random opticalfield can be measured and the phase distribution of the random opticalfield can be measured, thereby achieving high spatial resolution.

4. The measurement solution in the present invention is a Hanbury Brownand Twiss type experiment, and the high-order correlation has a robustfeature against turbulence.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a test optical path according to thepresent invention; and

FIG. 2 is a schematic flowchart according to the present invention.

Reference numerals: 1, to-be-tested light source; 2, reference lightsource; 3, quarter-wave plate; 4, beam splitter; 5, condensing element;6, light detector; 7, computer; and 8, reflector.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is further described below with reference to theaccompanying drawings and specific embodiments, to enable a personskilled in the art to better understand and implement the presentinvention. However, the embodiments are not used to limit the presentinvention.

Referring to FIG. 1 and FIG. 2, the present invention discloses a methodfor measuring a complex degree of coherence of a random optical field byusing a mutual intensity-intensity correlation, including the followingsteps:

S1. a test optical path including a quarter-wave plate 3, a beamsplitter 4, a condensing element 5, and a light detector 6 is built, alaser is used as reference light, where reference light source 2 emitsthe reference light, the reference light is modulated by thequarter-wave plate and then enters the beam splitter 4, at the same timea to-be-tested light source 1 emits to-be-tested light, the to-be-testedlight enters the beam splitter 4, the beam splitter 4 combines themodulated reference light and the to-be-tested light to obtain combinedlight, and the combined light passes through the condensing element 5 tobe imaged on the light detector 6. The condensing element 5 may be athin lens. Herein, to enable the reference light and the to-be-testedlight to be both emitted to the beam splitter 4, a reflector 8 may beadded to an incident optical path of the reference light or theto-be-tested light, to adjust the direction of the optical path. Thelight detector 6 is connected to a computer 7. The computer 7 storesinformation collected by the light detector 6 and performs computation.Herein, the to-be-tested light is random light.

S2. the quarter-wave plate 3 is rotated to enable the fast axis of thequarter-wave plate to be consistent with a polarization direction of thereference light, to obtain first combined light, and photographing andrecording light intensity distribution information I_(S) ⁽¹⁾(r) of thefirst combined light by using the light detector.

S3. the quarter-wave plate 3 is rotated to enable the slow axis of thequarter-wave plate to be consistent with the polarization direction ofthe reference light, to obtain second combined light, and photographingand recording light intensity distribution information I_(S) ⁽²⁾(r) ofthe second combined light by using the light detector.

S4. the reference light is blocked from reaching the beam splitter, andin this case, the to-be-tested light is not blocked, and light intensitydistribution information I(r) of the to-be-tested light is photographedand record by using the light detector.

S5. the to-be-tested light is blocked from reaching the beam splitter,and in this case, the reference light is not blocked, and lightintensity distribution information S_(r)(r) of the reference light isphotographed and record by using the light detector.

S6. the amplitude and phase of a complex degree of coherence of theto-be-tested light are calculated.

The “calculating the amplitude and phase of a complex degree ofcoherence of the to-be-tested light” specifically includes the followingsteps:

S61. a mutual correlation G_(S) ^((1,2))(r₁,r₂) between the lightintensity distribution information I_(S) ⁽¹⁾(r) of the first combinedlight and the light intensity distribution information I_(S) ⁽²⁾(r) ofthe second combined light is calculated.

Specifically, according to the definition of a second-order coherencematrix in the space-frequency domain proposed by Wolf et al., asecond-order statistical feature (including the coherence) of theto-be-tested light can be represented, by using a cross-spectral densityfunction, as:

W(r ₁ ,r ₂)=

E*(r ₁)E(r ₂)

  (1),

where E(r) represents a random electrical field at a point r in a space,a superscript asterisk represents a complex conjugate, an angle bracketrepresents ensemble averaging, and in this case, a complex spatialdegree of coherence between two points r₁ and r₂ in the space may bedefined as:

$\begin{matrix}{{{\mu\left( {r_{1},r_{2}} \right)} = \frac{W\left( {r_{1},r_{2}} \right)}{\sqrt{{S\left( r_{1} \right)}{S\left( r_{2} \right)}}}},} & (2)\end{matrix}$

where S(r)=W(r,r)=

I(r)

represents average light intensity of a random optical field at thepoint r in the space;

in the step of “rotating the quarter-wave plate to enable the fast axisof the quarter-wave plate to be consistent with a polarization directionof the reference light”, an electrical field of the reference lightobtained through modulation by using the quarter-wave plate is denotedas E_(r) ⁽¹⁾(r), in the step of “rotating the quarter-wave plate toenable the slow axis of the quarter-wave plate to be consistent with thepolarization direction of the reference light”, an electrical field ofthe reference light obtained through modulation by using thequarter-wave plate is denoted as E_(r) ⁽²⁾(r), and an electrical fieldof the to-be-tested light is denoted as E(r);

because E_(r) ⁽¹⁾(r) is an electrical field of the reference lightobtained when the fast axis of the quarter-wave plate coincides with thepolarization direction and E_(r) ⁽²⁾(r) is an electrical field of thereference light obtained when the slow axis of the quarter-wave platecoincides with the polarization direction, there is a phase differenceof

$\frac{\pi}{2}$

between the electrical fields E_(r) ⁽¹⁾(r) and E_(r) ⁽²⁾(r), that is:

$\begin{matrix}{{{\Delta\phi} = {{{{Arg}\left\lbrack {E_{r}^{(1)}(r)} \right\rbrack} - {{Arg}\left\lbrack {E_{r}^{(2)}(r)} \right\rbrack}} = \frac{\pi}{2}}},} & (3)\end{matrix}$

where Arg represents calculating the phase of a complex function;

an electrical field E_(S) ⁽¹⁾(r) of the first combined light and anelectrical field E_(S) ⁽²⁾(r) of the second combined light arerepresented as follows:

E _(S) ⁽¹⁾(r)=E(r)+E _(r) ⁽¹⁾(r)   (4),

E _(S) ⁽²⁾(r)=E(r)+E _(r) ⁽²⁾(r)   (5),

the mutual correlation between the light intensity distributioninformation I_(S) ⁽¹⁾(r) of the first combined light and the lightintensity distribution information I_(S) ⁽²⁾(r) of the second combinedlight is:

G _(S) ^((1,2))(r ₁ ,r ₂)=

I_(S) ⁽¹⁾(r ₁)I _(S) ⁽²⁾(r ₂)

  (6), and

according to theorem of Gaussian statistics, Formula (1) to Formula (5)are substituted into Formula (6), to obtain:

G _(S) ^((1,2))(r ₁ ,r ₂)=G _(S) ^((1,2))(r ₁ ,r ₂,Δϕ)=S _(S) ⁽¹⁾(r ₁)S_(S) ⁽²⁾(r ₂)+|W(r ₁ ,r ₂)|²+2√{square root over (S _(r) ⁽¹⁾(r ₁)S _(r)⁽²⁾(r ₂))}Re[e ^(iΔϕ) W(r ₁ ,r ₂)]  (7).

Re represents calculating the real part of the complex function. It isfound that the mutual intensity-intensity correlation G_(S)^((1,2))(r₁,r₂,Δϕ) is correlated to a phase difference Δϕ betweenreference optical paths, and the amplitude information and phaseinformation of the degree of coherence of the to-be-tested light arealso included in G_(S) ^((1,2))(r₁,r₂,Δϕ). For example: when Δϕ=0, thelast term in Formula (7) includes the real part information of thecomplex degree of coherence; and when

${{\Delta\phi} = \frac{}{2}},$

the last term in Formula (7) includes the imaginary part information ofthe complex degree of coherence. The phase difference between the tworeference optical paths can be controlled to obtain the real partinformation and the imaginary part information of the complex degree ofcoherence, so as to obtain the amplitude and phase of the complex degreeof coherence of a random light field. In addition, it is found fromFormula (7) that the mutual intensity-intensity correlation functionincludes background terms.

To remove background terms, the mutual intensity-intensity correlationof incoherent superposition of the reference light and the to-be-testedlight, that is, S62, is introduced.

S62. a mutual correlation G_(B) ^((1,2))(r₁,r₂) between the referencelight and the to-be-tested light is calculated.

$\begin{matrix}\begin{matrix}{{G_{B}^{({1,2})}\left( {r_{1},r_{2}} \right)} = \left\langle {\left\lbrack {{S_{r}^{(1)}\left( r_{1} \right)} + {I\left( r_{1} \right)}} \right\rbrack\left\lbrack {{S_{r}^{(2)}\left( r_{2} \right)} + {I\left( r_{2} \right)}} \right\rbrack} \right\rangle} \\{= \left. {{{S_{S}^{(1)}\left( r_{1} \right)}{S_{S}^{(2)}\left( r_{2} \right)}} +} \middle| {W\left( {r_{1},r_{2}} \right)} \middle| {}_{2}. \right.}\end{matrix} & (8)\end{matrix}$

S63. a difference value ΔG^((1,2))(r₁,r₂,Δϕ) between the two mutualcorrelations G_(S) ^((1,2))(r₁,r₂) and G_(B) ^((1,2))(r₁,r₂) iscalculated.

ΔG _(S) ^((1,2))(r ₁ ,r ₂,Δϕ)=G _(S) ^((1,2))(r ₁ ,r ₂,Δϕ)−G _(B)^((1,2))(r ₁ ,r ₂)=√{square root over (S _(r) ⁽¹⁾(r ₁)S _(r) ⁽²⁾(r₂))}Re[e ^(iΔϕ) W(r ₁ ,r ₂)]  (9).

S64. the difference value ΔG^((1,2))(r₁,r₂,Δϕ) between the two mutualcorrelations is analyzed, to obtain the amplitude and phase of thecomplex degree of coherence of the to-be-tested light.

For ΔG^((1,2))(r₁,r₂,Δϕ), a distribution Δϕ is taken to be 0 and

$\frac{\pi}{2},$

and at the same time the real part and the imaginary part of the complexdegree of coherence are obtained according to Formula (2):

$\begin{matrix}{{{{Re}\left\lbrack {\mu\left( {r_{1},r_{2}} \right)} \right\rbrack} = \frac{\Delta{G^{({1,2})}\left( {r_{1},r_{2},{{\Delta\phi} = 0}} \right)}}{2\sqrt{{S_{r}^{(1)}\left( r_{1} \right)}{S_{r}^{(2)}\left( r_{2} \right)}{S\left( r_{1} \right)}{S\left( r_{2} \right)}}}},} & (10) \\{and} & \; \\{{{{Im}\left\lbrack {\mu\left( {r_{1},r_{2}} \right)} \right\rbrack} = \frac{\Delta{G^{({1,2})}\left( {r_{1},r_{2},{{\Delta\phi} = \frac{\pi}{2}}} \right)}}{2\sqrt{{S_{r}^{(1)}\left( r_{1} \right)}{S_{r}^{(2)}\left( r_{2} \right)}{S\left( r_{1} \right)}{S\left( r_{2} \right)}}}},} & (11)\end{matrix}$

in the foregoing formulas, Im represents calculating the imaginary partof the complex function, and the amplitude and phase of the complexdegree of coherence of the to-be-tested light are obtained by usingFormula (10) and Formula (11).

In the invention, the light detector is a CCD or a CMOS. In thisembodiment, the used CCD is the professional camera of the modelGrasshopper GRAS-20S4M developed by the Point Grey Company. For specificparameters, the horizontal resolution is 1624, the vertical resolutionis 1224, and the frame frequency/row frequency is 30 fps. The CCD isconnected to a computer installed with software named Point Greyprovided by the Point Grey Company. The foregoing software is used forobserving and saving image information received by the CCD. After theCCD is connected to the computer, the software Point Grey is opened torecord and save the image information received by the CCD.

In the present invention, the reference light is emitted by ahelium-neon laser. The helium-neon laser generates fully coherent linepolarized light with a wavelength of 632.8 nanometers.

The to-be-tested light may be produced by using the solution in PatentNo. CN201410399805.8.

The foregoing embodiments are merely preferred embodiments used to fullydescribe the present invention, and the protection scope of the presentinvention is not limited thereto. Equivalent replacements or variationsmade by a person skilled in the art to the present invention all fallwithin the protection scope of the present invention. The protectionscope of the present invention is as defined in the claims.

What is claimed is:
 1. A method for measuring a complex degree ofcoherence of a random optical field by using a mutualintensity-intensity correlation, comprising steps of: building a testoptical path comprising a quarter-wave plate, a beam splitter, acondensing element, and a light detector, using a laser as referencelight, wherein the reference light is modulated by the quarter-waveplate and then enters the beam splitter, and to-be-tested light entersthe beam splitter at the same time, the beam splitter combines themodulated reference light and the to-be-tested light to obtain combinedlight, and the combined light passes through the condensing element tobe imaged on the light detector; rotating the quarter-wave plate toenable the fast axis of the quarter-wave plate to be consistent with apolarization direction of the reference light to obtain first combinedlight, and photographing and recording light intensity distributioninformation I_(S) ⁽¹⁾(r) of the first combined light by using the lightdetector; rotating the quarter-wave plate to enable the slow axis of thequarter-wave plate to be consistent with the polarization direction ofthe reference light to obtain second combined light, and photographingand recording light intensity distribution information I_(S) ⁽²⁾(r) ofthe second combined light by using the light detector; blocking thereference light, and photographing and recording light intensitydistribution information I(r) of the to-be-tested light by using thelight detector; blocking the to-be-tested light, and photographing andrecording light intensity distribution information S_(r)(r) of thereference light by using the light detector; and calculating theamplitude and phase of a complex degree of coherence of the to-be-testedlight.
 2. The method for measuring a complex degree of coherence of arandom optical field by using a mutual intensity-intensity correlationaccording to claim 1, wherein the “calculating the amplitude and phaseof a complex degree of coherence of the to-be-tested light” comprisesthe following steps: S61. calculating a mutual correlation G_(S)^((1,2))(r₁,r₂) between the light intensity distribution informationI_(S) ⁽¹⁾(r) of the first combined light and the light intensitydistribution information I_(S) ⁽²⁾(r) of the second combined light; S62.calculating a mutual correlation G_(B) ^((1,2))(r₁,r₂) obtained byadding the reference light and the to-be-tested light; S63. calculatinga difference value ΔG^((1,2))(r₁,r₂,Δϕ) between the two mutualcorrelations G_(S) ^((1,2))(r₁,r₂) and G_(B) ^((1,2))(r₁,r₂); and S64.analyzing the difference value ΔG^((1,2))(r₁,r₂,Δϕ) between the twomutual correlations, to obtain the amplitude and phase of the complexdegree of coherence of the to-be-tested light.
 3. The method formeasuring a complex degree of coherence of a random optical field byusing a mutual intensity-intensity correlation according to claim 1,wherein S61 comprises: according to a second-order coherence matrix inthe space-frequency domain, representing, by using a cross-spectraldensity function, a second-order statistical feature of the to-be-testedlight as:W(r ₁ ,r ₂)=

E*(r ₁)E(r ₂)

  (1), wherein E(r) represents a random electrical field at a point r ina space, a superscript asterisk represents a complex conjugate, an anglebracket represents ensemble averaging, and in this case, a complexspatial degree of coherence between two points r₁ and r₂ in the spacemay be defined as: $\begin{matrix}{{{\mu\left( {r_{1},r_{2}} \right)} = \frac{W\left( {r_{1},r_{2}} \right)}{\sqrt{{S\left( r_{1} \right)}{S\left( r_{2} \right)}}}},} & (2)\end{matrix}$ wherein S(r)=W(r,r)=

I(r)

represents average light intensity of a random optical field at thepoint r in the space; in the step of “rotating the quarter-wave plate toenable the fast axis of the quarter-wave plate to be consistent with apolarization direction of the reference light”, an electrical field ofthe reference light obtained through modulation by using thequarter-wave plate is denoted as E_(r) ⁽¹⁾(r), in the step of “rotatingthe quarter-wave plate to enable the slow axis of the quarter-wave plateto be consistent with the polarization direction of the referencelight”, an electrical field of the reference light obtained throughmodulation by using the quarter-wave plate is denoted as E_(r) ⁽²⁾(r),and an electrical field of the to-be-tested light is denoted as E(r);there is a phase difference of $\frac{\pi}{2}$ between the electricalfields E_(r) ⁽¹⁾(r) and E_(r) ⁽²⁾(r), that is: $\begin{matrix}{{{\Delta\phi} = {{{{Arg}\left\lbrack {E_{r}^{(1)}(r)} \right\rbrack} - {{Arg}\left\lbrack {E_{r}^{(2)}(r)} \right\rbrack}} = \frac{\pi}{2}}},} & (3)\end{matrix}$ wherein Arg represents calculating the phase of a complexfunction; an electrical field E_(S) ⁽¹⁾(r) of the first combined lightand an electrical field E_(S) ⁽²⁾(r) of the second combined light arerepresented as follows:E _(S) ⁽¹⁾(r)=E(r)+E _(r) ⁽¹⁾(r)   (4),E _(S) ⁽²⁾(r)=E(r)+E _(r) ⁽²⁾(r)   (5), the mutual correlation betweenthe light intensity distribution information I_(S) ⁽¹⁾(r) of the firstcombined light and the light intensity distribution information I_(S)⁽²⁾(r) of the second combined light is:G _(S) ^((1,2))(r ₁ ,r ₂)=

I_(S) ⁽¹⁾(r ₁)I _(S) ⁽²⁾(r ₂)

  (6), and according to theorem of Gaussian statistics, Formula (1) toFormula (5) are substituted into Formula (6), to obtain:G _(S) ^((1,2))(r ₁ ,r ₂)=G _(S) ^((1,2))(r₁ ,r ₂,Δϕ)=S _(S) ⁽¹⁾(r ₁)S_(S) ⁽²⁾(r ₂)+|W(r ₁ ,r ₂)|²+2√{square root over (S _(r) ⁽¹⁾(r ₁)S _(r)⁽²⁾(r ₂))}Re[e ^(iΔϕ) W(r ₁ ,r ₂)]  (7).
 4. The method for measuring acomplex degree of coherence of a random optical field by using a mutualintensity-intensity correlation according to claim 3, wherein S62comprises: $\begin{matrix}\begin{matrix}{{G_{B}^{({1,2})}\left( {r_{1},r_{2}} \right)} = \left\langle {\left\lbrack {{S_{r}^{(1)}\left( r_{1} \right)} + {I\left( r_{1} \right)}} \right\rbrack\left\lbrack {{S_{r}^{(2)}\left( r_{2} \right)} + {I\left( r_{2} \right)}} \right\rbrack} \right\rangle} \\{= \left. {{{S_{S}^{(1)}\left( r_{1} \right)}{S_{S}^{(2)}\left( r_{2} \right)}} +} \middle| {W\left( {r_{1},r_{2}} \right)} \middle| {}_{2}. \right.}\end{matrix} & (8)\end{matrix}$
 5. The method for measuring a complex degree of coherenceof a random optical field by using a mutual intensity-intensitycorrelation according to claim 4, wherein S63 comprises:ΔG _(S) ^((1,2))(r ₁ ,r ₂,Δϕ)=G _(S) ^((1,2))(r ₁ ,r ₂,Δϕ)−G _(B)^((1,2))(r ₁ ,r ₂)=√{square root over (S _(r) ⁽¹⁾(r ₁)S _(r) ⁽²⁾(r₂))}Re[e ^(iΔϕ) W(r ₁ ,r ₂)]  (9).
 6. The method for measuring a complexdegree of coherence of a random optical field by using a mutualintensity-intensity correlation according to claim 5, wherein S64comprises: for ΔG^((1,2))(r₁,r₂,Δϕ), taking a distribution Δϕ to be 0and $\frac{\pi}{2},$ and obtaining the real part and the imaginary partof the complex degree of coherence according to Formula (2):$\begin{matrix}{{{{Re}\left\lbrack {\mu\left( {r_{1},r_{2}} \right)} \right\rbrack} = \frac{\Delta{G^{({1,2})}\left( {r_{1},r_{2},{{\Delta\phi} = 0}} \right)}}{2\sqrt{{S_{r}^{(1)}\left( r_{1} \right)}{S_{r}^{(2)}\left( r_{2} \right)}{S\left( r_{1} \right)}{S\left( r_{2} \right)}}}},} & (10) \\{{{{Im}\left\lbrack {\mu\left( {r_{1},r_{2}} \right)} \right\rbrack} = \frac{\Delta{G^{({1,2})}\left( {r_{1},r_{2},{{\Delta\phi} = \frac{\pi}{2}}} \right)}}{2\sqrt{{S_{r}^{(1)}\left( r_{1} \right)}{S_{r}^{(2)}\left( r_{2} \right)}{S\left( r_{1} \right)}{S\left( r_{2} \right)}}}},} & (11)\end{matrix}$ in the foregoing formulas, Im represents calculating theimaginary part of the complex function, and obtaining the amplitude andphase of the complex degree of coherence of the to-be-tested light byusing Formula (10) and Formula (11).
 7. The method for measuring acomplex degree of coherence of a random optical field by using a mutualintensity-intensity correlation according to claim 1, wherein the lightdetector is a charge-coupled device (CCD) or a complementarymetal-oxide-semiconductor (CMOS).
 8. The method for measuring a complexdegree of coherence of a random optical field by using a mutualintensity-intensity correlation according to claim 1, wherein thereference light is emitted by a helium-neon laser.